It is desirable to afford increased automation in four-wheel drive vehicles and in particular to permit selective engagement and disengagement of half of the drive train so as to establish four-wheel drive or two-wheel drive under the complete control of the operator. This is especially important if the vehicle is in motion and it is necessary to convert between two-wheel and four-wheel drive while the vehicle is in motion or "on the fly". Typically, in the past, clutch assemblies have required that the vehicle operator stop the vehicle, then reverse it as a preliminary to disengaging or converting from four-wheel drive to two-wheel drive. However, with the increased need for four-wheel drive adhesion on slippery turnpikes or busy highways, the desirability of engaging or disengaging on the fly is apparent.
Numerous approaches have been taken in the past to permit dynamic engagement or disengagement on the fly. One approach has been to employ a supplementary or remote power source to actuate a drive gear, or to employ pneumatic systems whereby vacuum or fluid pressure motors or valves cause the drive gear to engage and subsequently disengage by spring pressure when the vacuum or fluid pressure is removed. The same type of system employing hydraulic pressure and an outside power source similar to the brake system has been employed in an effort to accomplish the same end. However, the approaches taken in the past have for the most part required that the pressure or pressure drop be maintained across the valve or motor actuators throughout the extended time of four-wheel drive mode operation as well as to maintain sufficient pressure to compress the disengaged spring member.
The above and other approaches which employ supplementary or outside power sources have necessitated the use of conduits to transmit power along the underside of the vehicle to or near the axles and wheels at locations normally subjected to abuse, striking surface obstructions as well as being subject to flying debris, mud or water and it is extremely difficult to effectively armor or protect the conduits along the axle or suspension area. Furthermore, in using supplementary power sources, it is extremely important to provide a system which will "fail-safe" into the four-wheel drive mode as opposed to fail-safing into the two-wheel drive mode.
Another important consideration is that the supplementary power source adds considerably to the original cost and maintenance of the vehicle but seldom can be retrofit to existing vehicles and generally requires considerable design changes to the wheel and drive train components.
Still further, units which rely upon cam members to effect engagement to ground or a fixed locking member in the course of converting to four-wheel drive must rely upon some form of a drive spring which remains engaged with the locating cams at the conversion into four-wheel drive. Specifically, the approaches taken had been to employ either a wrap spring engagement to ground and which spring is released after camming but is continuously driven in the four-wheel drive mode, or a high friction cone-shaped clutch which is also continuously driven and remains engaged during four-wheel drive mode. Still further, a common practice has been to rely upon a large spring to effect disengagement between the driver and receiving gears in converting from four-wheel drive mode back into two-wheel drive mode. It is highly desirable to avoid reliance upon a spring member either to effect engagement to ground or to disengage the drive and receiver gears and in such a way as to effect complete disengagement from ground both during four-wheel drive and two-wheel drive mode so that when the mechanism is shifted to two-wheel drive mode, spin-down of the rotating components can be achieved.